The present invention generally relates to semiconductor devices and more particularly to a semiconductor device having a ferroelectric film.
Semiconductor storage devices called DRAMs or SRAMs are used extensively as high speed main memory device in information processing apparatuses including computer. However, these devices are volatile storage devices and the information stored therein is lost when the power supply is turned off. In view of this, non-volatile magnetic disk devices have been used as large-capacity auxiliary storage devices for storing programs and data.
However, a magnetic disk unit is mechanically fragile and has problems such as large electric power consumption and slow access speed at the time of reading and writing information.
On the other hand, use of EEPROMs or flash memory devices, which accumulate information in a floating gate electrode in the form of electric charges, is increasing recently for the purpose of non-volatile auxiliary storage devices. Particularly, a flash memory device has a cell construction similar to that of a DRAM and can easily form an integrated circuit of large integration density. Thus, flash memory devices are expected for large capacity storage devices comparable to magnetic disk units.
With EEPROMs or flash memory devices, on the other hand, writing of information is carried out by injection of hot electrons to a floating gate electrode through a tunneling insulation film. Thus, there have been problems that a long time is needed to write information and that the tunneling insulation film tends to cause degradation when writing and erasing of information is repeated. When the tunneling insulation film is degraded, writing or erasing operation becomes unstable and unreliable.
Meanwhile, there is proposed a ferroelectric storage device (referred to hereinafter as FeRAM) that stores information in a ferroelectric film in the form of spontaneous polarization.
With such FeRAMs, each memory cell transistor is formed of a single MOS transistor similarly to the case of a DRAM, except that the dielectric film in the memory cell capacitor is replaced by a ferroelectric such as PZT (Pb (Zr,Ti)O3), PLZT ((Pb,La)(Zr,Ti)O3), SBT (SrBi2Ta2O3), SBTN (SrBi2(Ta,Nb)2O3), and the like. Thus, an FeRAM has a construction suitable for high-density integration.
Further, because an FeRAM controls the spontaneous polarization of the ferroelectric capacitor by application of an electric field, an FeRAM has an advantageous feature of very large writing speed, which reaches as large as 1000 times or more as compared with the case of using an EEPROM or a flash memory device, in which writing is achieved by injection of hot electrons. Further, an FeRAM has an advantageous feature of low electric power consumption, which can be reduced to 1/10 times or less as compared with the case of using an EEPROM or a flash memory device.
Further, in view of elimination of tunneling oxide film, an FeRAM has an advantageous feature of long lifetime, and thus, it is excepted that the number of rewriting information can be increased by 100,000 times as large as in the case of repeating rewriting of information with a flash memory device.
Patent Reference 1
Japanese Laid-Open Patent Application 11-219922 official gazette